Silver composition



Dec. 17, 1963 G. H. SISTARE ETAL SILVER COMPOSITION Filed Aug. 24, 1960FIG. I

Plain silver-cadmium alloy 400 diameters magnification FIG. 2

Silver-cadmium alloy with additive 400 diameters magnification INVENTORSGEORGE H. SISTARE (LDICKEY COXE Quin. Eluowb Wnkow3M-n A inf-a,

ATTORNEYS 3,114,631 HIVER COMPOSITION George H. Sistare and Charles D.(Ioxe, Fairfield, Conm, assignors to Handy & Harman, New York, N.Y., acorporation of New York Filed Aug. 24, 1960, Ser. No. 51,671 6 Claims.(Cl. 75173) This invention relates to alloys for use in makingelectrical contacts for switches, relays, circuit breakers and the like.More particularly, it relates to silver-cadmium alloys which can beinternally oxidized without adverse effect owing to the presence in thealloy of a small amount of silicon or aluminum or both. The inventioncontemplates providing the alloy in either unoxidized or internallyoxidized form, and it further contemplates the provision of cold-Workedelectrical contacts formed from the internally oxidized alloy and amethod of making such contacts.

Cadmium oxide-silver compositions have long been recognized as superiormaterials for use in making electrical contacts of the type described.Powder metallurgy was originally relied upon for forming the desiredcombination of metal and metal oxide, but it is the usual practice nowto cold-form the finished parts from silver-cadmium alloy wire and theninternally oxidize them to provide a silver matrix in which cadmiumoxide is dispersed. This method is by no means completely satisfactory,however, because the finished contacts are very often brittle and marredby longitudinal surface cracks as a result of the final oxidation step.Also, the process of internal oxidation must be carried out by themanufacturer of the contacts rather than by the supplier of thesilver-cadmium wire.

The ultimate cause of the brittleness and longitudinal surface crackscharacteristic of conventional internally oxidized electrical contactsis not known with complete certainty, but it appears that the plateletform in which the cadmium oxide appears in the silver matrix is acontributing factor. During oxidation, the cadmium oxide crystals growrapidly at the interfaces of the relatively larger crystals of silver toform fiat platelets or flakes, and so produce dominant planes ofcleavage in the structure at which later applied internal or externalstresses tend to cause failure by cracking. The internal oxidationprocess itself sets up severe internal stresses (primarily hoopstresses) which produce the characteristic cracks along these planes ofclevage. These stresses are due to the fact that as oxidation proceedsinwardly from the surface of the part to the core, it is accompanied byvolumetric expansion; and as the interior of the alloy oxidizes andexpands it subjects the previously oxidized outer layer to stresseswhich cause cracks to open at the surface along the planes defined bythe platelets of cadmium oxide.

It is the primary purpose of the present invention to provide asilver-cadmium alloy for use in making electrical contacts which is notsusceptible to such surface cracking when internally oxidized and whichis even sufficiently ductile and workable after oxidation to be oxidizedwhile still in the form of wire and later fabricated by cold-forminginto finished contacts.

These objects are achieved according to the invention by a newelectrical contact alloy of from 2 to 20 percent by weight of cadmium,from 4 to 18X l percent by weight of at least one element taken from thegroup consisting of aluminum and silicon, and the balance essentiallyall silver. The invention also provides the new alloy in the internallyoxidized condition. It then includes the oxides of cadmium and of saidelement (aluminum or silicon) in a silver matrix, the cadmium oxidebeing characteristically in the form of substantially sphertates atentice ical nodules substantially uniformly dispersed throughout thematrix, rather than platelets.

Electrical contacts are made in accordance with the invention byinternally oxidizing an alloy of from 2 to 5 20 percent by weight ofcadmium, from 4 to 18 10 percent by weight of at least one element takenfrom the group consisting of aluminum and silicon, and the balanceessentially silver, and thereafter cold-forming the alloy intoelectrical contacts.

The invention is described below with reference to the accompanyingdrawings, in which FIG. 1 shows the microstructure of an internally oxidized alloy of silver and cadmium; and

FIG. 2 shows the microstructure of an internally oxidized silver-cadmiumalloy containing silicon or alumi num in the percentages contemplated bythe invention.

It has been discovered that by the addition of small amounts of siliconor aluminum or both to a silver-cadmium alloy as described, theresulting cadmium oxide crystals form in substantially sphericalnodules, not platelets or flakes, which are substantially uniformlydispersed throughout the matrix. As a result the ductility of theoxidized alloy is considerably improved. The distinct nodular form ofthe cadmium oxide crystals which typifies the new alloy appears toresult from nucleation of the crystallizing cadmium oxide by previouslyformed evenly dispersed small crystals of silicon or aluminum oxide.This even dispersion and prior formation of the silicon or aluminumoxide crystals results from the fact that low concentrations of bothsilicon and aluminum are in solid solution in the silver-cadmium alloybefore oxidation, and their oxides have significantly higher heats offormation that cadmium oxide. Hence, the silicon or aluminum oxidizesfirst as the alloy is subjected to oxidation, and provides uniformlydispersed crystal nuclei about which the cadmium oxide crystals form. Itis clear, therefore, that the solid solubility in a silver-cadmium alloyand the higher heat of formation of their oxides than cadmium oxide areproperties of both silicon and aluminum which contribute to theremarkable success of the new alloy.

The advantages of the new alloy are shown clearly in the table below,wherein a conventional internally oxidized silver-cadmium alloycontaining 9% cadmium is compared to similar alloys which are modifiedby the addition of aluminum and silicon in accordance with theinvention. FIG. 1 and 2 of the accompanying drawing are referred to inthe table to show the microstructure of the old and new alloysrespectively.

Weight Ultimate Elou- Freedom Con- Additive Percent Strength, gation,From tact Crystal 10- 1,000 Percent Surface Heads Structure p.s.i.Cracks 0 31-35 21-25 Poor Poor Fig. 1. 5. 5 32-36 16-20 Good Goo Fig. 2.11 32-36 16-20 do do Fig. 2. 5.5 B l-38 16-20 o do Fig.2. D 11 34-3816-20 do do Fig. 2. Aluminum 22 30-34 3-7 Poor Poor Fig. 2. Silicon 2231-35 9-13 do do. Fig.2.

the fine nodular form of the cadmium oxide, provides considerably moreductility and workability than in the alloy of FIG. 1. As mentionedpreviously, this uniformity in the FIG. 2 alloy is believed to be causedby nucleation of the cadmium oxide by crystals of aluminum or siliconoxide, about which the cadmium oxide crystallizes in intricatelydispersed spheroidal form. Because of their higher heats of formation,the oxides of aluminum and silicon are formed closer behind theadvancing oxidation front, during oxidation of the alloy, than doescadmium oxide; and because the silicon or aluminum content is in solidsolution, their oxides are formed very uniformly dispersed throughoutthe alloy.

It is to be noted that aluminum and silicon do not achieve thisadvantageous structural modification of the silver-cadmium alloy onaccount of their deoxidizing properties. Indeed, many deoxidizers, suchas lithium and phosphorous (which are among these most commonly used),not only do not improve the alloy structure but actually reduce itsductility and workability.

Following is an example of the preparation of an electrical contact inaccordance with the invention: An alloy of 91 percent by weight ofsilver and 9 percent by weight of cadmium was melted by conventionalmeans and heated to about 2000 F. Silicon metal, in the form of asilicon-silver master alloy, was added to the silver-cadmium melt in anamount equivalent to 12x10 percent by weight. The melt was then cast,rolled into rods, and drawn into wire of 0.120 inch in diameter. Thiswire was internally oxidized by heating at 1550 F. in air for 72 hours.After such oxidation, the wire was sufficiently ductile to be cold drawnto .090 inch in diameter. It was then annealed at 1000 F. and was colddrawn to .088 inch in diameter. Finally, it was cold-headed into therivet form typical of an electrical contact. The finished contacts werehard in the headed portion and free of cracks.

In another example of the invention, an alloy of 91 percent by Weight ofsilver and 9 percent by weight of cadmium was prepared and heated to 200P. Then 8 x" percent by weight of aluminum was added to the melt and themetal was cast, rolled, and drawn into wire 0.120 inch in diameter.After internal oxidation by heating at 1550 F. in air for 72 hours, thewire was cold drawn to .090 inch in diameter, annealed at 1000 F andcold finished to .088 inch in diameter. The wire was then cold-headed toform rivets which were hard in the headed portion and free from surfacecracks.

It is apparent from these examples that silicon and aluminum are bothhighly effective for making the new contact alloys. In certaininstances, however, silicon may be preferred because it does not produceas much dross during melting as does aluminum. Moreover, silicon ispreferred for production because its presence in casting shops wheresilver alloys are being melted is unobjectionable, whereas aluminum issometimes sought to be excluded from such shops because it is anobjectionable contaminant in other silver alloys being produced.

Any of the nominal compositions of silver-cadmium oxide which have beencommercially produced heretofor may be prepared in accordance with theinvention. Such compositions generally contain from 2 to percent byweight of cadmium and the balance essentially silver. The most commonlyused materials, however, are these nominally containing 10 to 15 percentby weight of cadmium. Impurities normally'present in silver-cadmiumalloys, or silver-cadmium oxide compositions, may be present withoutharmful effect in the alloys of this invention.

The silicon and aluminum additives which modify the alloy structure inaccordance with the invention may be used singly or together. When usedjointly their total amount preferably is in the same range as for eitherone alone when used singly, that is at least 4 10 percent by weight butnot more than 18x10 percent by weight of the alloy. In most instances,the preferred range for the silicon or aluminum is from 5 to 12x10percent by weight.

Less than about *4 10 percent of silicon or aluminum or their combinedtotal is not suficient for the purposes of this invention, and more thanabout 18x10- percent is harmful to the properties of the alloy as shownin the last two items of the table in column 2.

We claim:

1. An internally oxidized alloy produced by the internal oxidation of asolid solution alloy consisting essentially of cadmium in an amount offrom 2% to 20%, by weight, from 4 to l8 10 percent, by Weight, of atleast one element from the group consisting of aluminum and silicon andthe balance silver, said cadmium being substantially in the form ofspherical oxide nodules substantially uniformly dispersed throughout thesilver matrix.

2. An electrical contact element composed of an alloy according to claim1 in the plastically-deformed cold worked condition.

3. An internally oxidized alloy according to claim 1 in which thecadmium is present in an amount of from about 10 to 15 percent, byweight, and the element of said group is aluminum and is present in thealloy in amount of from 5 to 12 l0 percent by weight.

4. An electrical contact element composed of an alloy according to claim3 in the plastically-deformed coldworked condition.

5. An internally oxidized alloy according to claim 1 in which cadmium ispresent in the alloy in amount of from 10 to 15 percent, by weight, andthe element from said group is silicon and is present in the alloy in anamount from 5 to 12 1O- percent, by weight.

6. An electrical contact element composed of the internally oxidizedalloy of claim 5 in the plasticallydeformed cold-worked condition.

References Cited in the file of this patent UNITED STATES PATENTS2,258,492 Hensel et al Oct. 7, 1941 2,396,101 Hensel et a1 Mar. 5, 19462,796,346 Stumbock June 18, 1957 2,932,595 Pfiurnm Apr. 12, 1960 FOREIGNPATENTS 611,813 Great Britain Nov. 4, 1948 542,630 Canada June 25, 1957

1. AN INTERNALLY OXIDIZED ALLOY PRODUCED BY THE INTERNAL OXIDATION OF ASOLID SOLUTION ALLOY CONSISTING ESSENTIALLY OF CADMIUM IN AN AMOUNT OFFROM 2% TO 20%, BY WEIGHT, FROM 4 TO 18X10-3 PERCENT, BY WEIGHT, OF ATLEAST ONE ELEMENT FROM THE GROUP CONSISTING OF ALUMINUM AND SILICON ANDTHE BALANCE SILVER, SAID CADMIUM BEING SUBSTANTIALLY IN THE FORM OFSPHERICAL OXIDE NODULES SUBSTANTIALLY UNIFORMLY DISPERSED THROUGHOUT THESILVER MATRIX.